The invention is especially useful with metal halide arc discharge lamps used for general illumination which comprise an arc tube of quartz or fused silica having electrodes at each end and containing a fill of mercury, metal halide and an inert gas for starting purposes. In common with most high intensity discharge lamps used for general illumination, such lamps are provided with an outer envelope or jacket made of glass which encloses the arc tube. The jacket is either evacuated or filled with an inactive gas and keeps air away from the seals and metal inleads which otherwise would oxidize at the operating temperature. It gives mechanical protection, and serves as a heat conservator and as a short wave ultraviolet filter. The jacket generally comprises a bulbous or ellipsoidal main portion which is extended at one end into a tubular neck portion to which is attached a screw type base for accommodating the lamp in a standard socket. The common practice has been to mount the arc tube axially within the jacket and this of course facilitates passing the arc tube mount or frame through the neck in lamp manufacture.
Vertical operation of the arc tube is the preferred mode resulting in higher efficiency and longer life. The discharge in a metal halide lamp is a constricted arc which extends along the axis or midline of the arc tube so long as the arc tube is vertical. If the arc tube is inclined out of the vertical, and all the more so if it is made horizontal, internal convection currents affect the discharge and displace it from the axis. In a horizontal arc tube, the discharge is bowed up and may even contact the upper wall. This means that part of the arc tube where the discharge is in close proximity to the upper wall is overheated while the lower extremities are underheated. The partial overheating is responsible for poorer maintenance and shorter life, while the partial underheating reduces the vapor pressure of the metal halides and causes lower efficacy and poorer color rendition. The end result is that efficacy and life are reduced from 10 to 15% when a conventional metal halide lamp is operated with the arc tube horizontal instead of vertical.
While vertical operation is most common, there are many installations wherein considerations of space or convenience and economy require that the outer envelope of the lamp be mounted horizontally. In recent years, primarily as a result of improvements in color rendition, metal halide lamps have found increasing use indoors and in applications where the ceiling height is limited. Fixtures for such applications mount the outer envelope horizontally in order to save space. This has increased the demand for metal halide lamps able to operate without reduction in efficacy or life when the outer envelope is horizontal.
Mounting the arc tube transversely to the axis of the jacket is difficult because the arc tube is longer than the diameter of the neck. One scheme for so doing which is currently in commercial use is described and claimed in copending application Ser. No. 158,932 filed June 12, 1980 by Phillipp et al, now U.S. Pat. No. 4,341,975, and assigned to the same assignee as the present invention. It involves inserting into the jacket a folded incompletely assembled mount assembly in which the arc tube is hinged to a long support rod extending from the stem, and a short support rod trails from the arc tube. After insertion, the arc tube is transversely erected and the short rod is welded inside the jacket to its lead-in wire sealed through the stem. In smaller sizes of lamp, the jacket has a narrower neck and that makes interior welding extremely difficult. Also when this scheme is used with lamps in which the jacket is internally coated with a phosphor or diffuser, a careless operator may scratch the coating badly.